Does the spec preclude objects being movable by the GC?

[Ben Davis]

Unless I'm mistaken, this page:

http://dlang.org/garbage.html

states that it's safe to use a union to share storage with a pointer, e.g.

union U { void* ptr; int value }

But that wouldn't be safe if the GC ever moved objects. Should the page 
be tweaked? Especially as further down on that page, it says that a 
moving garbage collector can be implemented if one follows the rules.